Broadband microstrip filter apparatus having inteleaved resonator sections

Abstract

A broadband microstrip filter structure employs a first transmission line which basically consists of a series of capacitor plates which extend between a dielectric layer from a first location to a second location and each of the series of plates are connected together with a top plate on the top surface of the dielectric and a bottom plate located a given distance beneath the dielectric layer. A second transmission line alternates between the top and bottom layers of the dielectric and consists of a second series of capacitive plates whereby each capacitor is connected to an adjacent capacitor with the top plate of the first connecting to the bottom plate of the second and the bottom plate of the first connecting to the top plate of the second and so on. This pattern is repeated so that the conductor path alternates from the top to the bottom plate. Various capacitors are selected to provide predetermined length resonators while various other capacitors are provided to provide coupling sections. In this configuration each of the above-noted lines provide switching from the top to the bottom so that each conductor averages the same distance from the ground plane insuring identical impedances in each line. The structure allows tight coupling and enables the even and odd mode phase velocity differences to be compensated for due to the fact that the odd mode travels between the two conductors and the even mode travels between the conductor and the ground plane. In this manner the odd mode travels faster but further, thus the even mode and the odd mode move down the structure in synchronism.

What is claimed is: 
     
       1. A broadband microstrip filter apparatus, comprising: a microstrip structure including a ground plane, said ground plane having a dielectric disposed thereon, said dielectric having a top surface;   at least one first conductive line having a first plurality of conductive areas and a second plurality of conductive areas, said first conductive areas being located on said top surface of said dielectric and said second conductive areas being located a given distance beneath said top surface, each separate first and second conductive area having a leading edge and a trailing edge and wherein each of said first conductive areas has a respective trailing edge connected to the leading edge of an adjacent second conductive area, and each of said adjacent second conductive areas has a respective trailing edge connected to the leading edge of a next adjacent first conductive area, all of said first plurality of conductive areas being thereby connected with all of said second plurality of conductive areas to constitute said first conductive line having a square wave pattern;   at least one second conductive line having a third plurality of conductive areas and a fourth plurality of conductive areas, said third conductive areas being located on said top surface of said dielectric and said fourth conductive areas being located said given distance beneath said top surface, each separate third and fourth conductive area having a leading edge and a trailing edge and wherein each of said third conductive areas has a respective trailing edge connected to the leading edge of an adjacent fourth conductive area, and each of said adjacent fourth conductive areas has a respective trailing edge connected to the leading edge of a next adjacent third conductive area, all of said third plurality of conductive areas being thereby connected with all of said fourth plurality of conductive areas to constitute said second conductive line having a square wave pattern, said conductive areas of said first conductive line being disposed relative to said conductive areas of said second conductive line to constitute an interlace pattern, therebetween said third plurality and said second plurality of conductive areas being thereby constituted as a first plurality of capacitors wherein each one of said third conductive areas of said second line constitutes a respective top capacitive plate and a respective one of said second conductive areas of said first line constitutes an associated bottom capacitive plate, said first plurality and said fourth plurality of conductive areas being thereby constituted as a second plurality of capacitors wherein each one of said first conductive areas of said first line constitutes a respective top capacitive plate and a respective one of said fourth conductive areas of said second line constitutes an associated bottom capacitive plate, whereby said first and said second lines are constituted as resonator sections each including a given number of said capacitors with each number of said capacitors of a line length of a fractional wavelength at a frequency of an input microwave signal applied to said filter thereby constituting said microstrip filter apparatus; and   wherein said input microwave signal is applied to said microstrip filter apparatus and propagates along said first and said second lines, said microwave signal having an odd mode wave propagating between said first and said second lines and an even mode wave propagating between said first line and said ground plane and between said second line and said ground plane, respectively, and whereupon said even and said odd mode waves travel in synchronism along said first and said second lines of said microstrip filter apparatus.   
     
     
       2. The filter apparatus according to claim 1 wherein said fractional wavelength line length of said resonator sections is a line length of a half wavelength at said frequency of said input microwave signal. 
     
     
       3. The filter apparatus according to claim 2 wherein said means for coupling resonator sections together includes a given number of said capacitors formed by said first and second lines having a length of a quarter wave at said microwave frequency to couple one resonator to another. 
     
     
       4. The filter apparatus according to claim 1 further including an input transmission line having one end coupled to said first line for accepting said input microwave signal applied to said filter. 
     
     
       5. The filter apparatus according to claim 4 wherein said input transmission line is a microstrip line disposed on said microstrip structure. 
     
     
       6. The filter apparatus according to claim 4 further including an output transmission line having one terminal coupled to said first line for providing an output signal for said filter. 
     
     
       7. The filter apparatus according to claim 6 wherein said output transmission line is a microstrip line disposed on said microstrip structure. 
     
     
       8. The filter apparatus according to claim 6 including output coupling means for coupling said output transmission line to said first and second lines. 
     
     
       9. The filter apparatus according to claim 8 wherein said output coupling means includes a quarter wave length capacitive structure. 
     
     
       10. The filter apparatus according to claim 4 including input coupling means for coupling said input transmission line to said first and second lines. 
     
     
       11. The filter apparatus according to claim 10 wherein said input coupling means includes a quarter wave length capacitive structure. 
     
     
       12. The filter apparatus according to claim 1 wherein said microstrip substrate is comprised of GaAs. 
     
     
       13. The filter apparatus according to claim 1 wherein said dielectric between said first and fourth areas is of a given thickness according to the amount of coupling desired. 
     
     
       14. The filter apparatus according to claim 1 wherein said dielectric between said second and third areas is of a given thickness according to the amount of coupling desired. 
     
     
       15. The filter apparatus according to claim 1, wherein said filter apparatus contains a center longitudinal axis, said first and said second lines being offset from one another transversely from said center longitudinal axis, wherein respective ones of said first and said second conductive areas of said first line partially overlap associated ones of said third and said fourth conductive areas of said second line, whereby said top capacitor plates are offset from said bottom capacitor plates by a given amount of overlap to thereby control the amount of coupling between said first and said second lines. 
     
     
       16. The filter apparatus according to claim 1, wherein said first and said fourth conductive areas are disposed relative to one another so that said leading and trailing edges of each of said first conductive areas are essentially coincident respectively with said leading and trailing edges of each of said fourth conductive areas. 
     
     
       17. The filter apparatus according to claim 1, wherein said second and said third conductive areas are disposed relative to one another so that said leading and trailing edges of each of said second conductive areas are essentially coincident respectively with said leading and trailing edges of each of said third conductive areas. 
     
     
       18. The filter apparatus according to claim 1, wherein said first and said fourth and said second and said third conductive areas, respectively, are disposed relative to one another so that said leading and trailing edges of each of said first conductive areas are essentially coincident respectively with said leading and trailing edges of each of said fourth conductive areas and said leading and trailing edges of each of said second conductive areas are essentially coincident respectively with said leading and trailing edges of each of said third conductive areas.